The peptidyl-prolyl cis-trans isomerases (PPIases), or rotamases, are a family of ubiquitous enzymes that catalyze the cis/trans isomerization of the peptide bond on the N-terminal side of proline residues in proteins (Hunter, Cell 92:141-142, 1998). PPIases are divided into three classes, cyclophilins (Cyps), FK-506 binding proteins (FKBPs) and the Pin1/parvulin class.
Cyclophilins and FKBPs are distinguished by their ability to bind the clinically immunosuppressive drugs cyclosporin and FK506, respectively (Schreiber, Science 251:283-7, 1991; Hunter, supra). Upon binding of these drugs, there are two common outcomes: inhibition of the PPIase activity and inhibition of the common target calcineurin. The inhibition of calcineurin phosphatase activity prevents lymphocytes from responding to antigen-induced mitogenic signals, thus resulting in immunusuppression. However, the inhibition of the PPIase activity is apparently unrelated to the immunosuppressive property of the drug/PPIase complexes. Even more surprisingly, deletion of all 8 known cyclophilins and 4 FKBPs in the same cells does not result in any significant phenotype (Dolinski et al., Proc. Natl. Acad. Sci. USA 94:13093-131098, 1997).
In contrast, members of the Pin1/parvulin class of PPIases bind neither of these immunosuppressive drugs, and are structurally unrelated to the other two classes of PPIases. Known members of the Pin1/parvulin class include Pins1-3 (Lu et al., Nature 380; 544-547, 1996), Pin-L (Campbell et al., Genomics 44:157-162, 1997), parvulin (Rahfeld, et al., Proc. Natl. Acad. Sci. USA 93:447-451, 1996) and Ess1/Pft1 (Hanes et al., Yeast 5:55-72, 1989; and Hani, et al. FEBS Letts 365:198-202, 1995).
Pin1 (Accession number: AAC50492) is a highly conserved protein that catalyzes the isomerization of only phosphorylated Ser/Thr-Pro bonds (Rananathan, R. et al. 1997 Cell 89:875-86; Yaffe, et al. 1997, Science 278:1957-1960; Shen, et al. 1998, Genes Dev. 12:706-720; Lu, et al. 1999, Science 283:1325-1328; Crenshaw, et al. 1998, Embo J. 17:1315-1327; Lu, et al. 1999, Nature 399:784-788; Zhou, et al. 1999, Cell Mol. Life. Sci. 56:788-806). In addition, Pin1 contains an N-terminal WW domain, which functions as a phosphorylated Ser/Thre-Pro binding module (Sudol, M. (1996) Prog. Biophys. Mol. Biol. 65:113-32). This phosphorylation-dependent interaction targets Pin1 to a subset of phosphorylated substrates, including Cdc25, Wee 1, Myt1, Tau-Rad4, and the C-terminal domain of RNA polymerase II large domain (Crenshaw, D. G., et al. (1998) Embo. J. 17:1315-27; Shen, M. (1998) Genes Dev. 12:706-20; Wells, N. J. (1999) J. Cell. Sci. 112: 3861-71).
The specificity of Pin1 activity is essential for cell growth; depletion or mutations of Pin1 cause growth arrest, affect cell cycle checkpoints and induce premature mitotic entry, mitotic arrest and apoptosis in human tumor cells, yeast or Xenopus extracts (Lu, et al. 1996, Nature 380:544-547; Winkler, et al. 2000, Science 287:1644-1647; Hani, et al. 1999. J. Biol. Chem. 274:108-116). In addition, Pin1 is dramatically misregulated in human cancer samples. Moreover, inhibition of Pin1 by various approaches, including Pin1 antisense polynucleotides or genetic depletion, kills human and yeast dividing cells by inducing premature mitotic entry and apoptosis. Further, Pin1 has been shown to be involved with the progression of neurodegenerative diseases such as Alzheimer's' disease.
Thus, Pin1-catalyzed prolyl isomerization regulates the conformation and function of these phosphoprotein substrates and facilitates dephosphorylation because of the conformational specificity of some phosphatases. Thus, Pin1-dependent peptide bond isomerization is a critical post-phosphorylation regulatory mechanism, allowing cells to turn phosphoprotein function on or off with high efficiency and specificity during temporally regulated events, including the cell cycle (Lu et al., supra).